The value of "low-priority-updates" option and the LOW PRIORITY
prefix was taken into account at parse time.
This caused triggers (among others) to ignore this flag (if
supplied for the DML statement).
Moved reading of the LOW_PRIORITY flag at run time.
Fixed an incosistency when handling
SET GLOBAL LOW_PRIORITY_UPDATES : now it is in effect for
delayed INSERTs.
Tested by checking the effect of LOW_PRIORITY flag via a 
trigger.


include/thr_lock.h:
  Bug #26162: moved reading of the LOW PRIORITY flag at run time
mysql-test/r/trigger.result:
  Bug #26162: test case
mysql-test/t/trigger.test:
  Bug #26162: test case
sql/set_var.cc:
  Bug #26162: fixed the handling of the "low-priority-updates" option
sql/sql_base.cc:
  Bug #26162: moved reading of the LOW PRIORITY flag at run time
sql/sql_yacc.yy:
  Bug #26162: moved reading of the LOW PRIORITY flag at run time